This invention relates to a method of fluidized catalytic cracking. More particularly, it relates to a method of minimizing post-riser cracking in the fluidized catalytic cracking (FCC) of hydrocarbons by increasing the rapidity of separation of the cracked products from the catalyst. It also relates to methods of removing the products from the reactor vessel quickly. In conventional FCC processes, separation of catalyst and reaction products is slow and product residence time in the reactor vessel is long compared with the FCC riser contact time of 1 to 4 seconds. Consequently, a large amount of post-riser cracking occurs, typically leading to 4% or more of non-selective conversion. This invention achieves rapid, efficient separation by means of a U-turn inertial separator.
Fluidized cracking of petroleum fractions is one of the major refining methods to convert crude petroleum oil to useful products such as fuels for internal combustion engines. In such fluidized catalytic cracking, (known popularly as "FCC") high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely divided solid catalyst particles in an elongated riser or transfer line reactor. The transfer line is usually in the form of a riser tube and the contacting time is on the order of a few seconds, say from 0.5 to 8 seconds, and generally not over about 4 seconds. During this short period, catalysts at temperatures in the range from about 1100.degree. F. to 1400.degree. F. are contacted with a hydrocarbon feedstock which is frequently a vacuum gas oil, cycle oil or the like, heated to a temperature of about 300.degree. to 800.degree. F. The reaction is one of essentially instantaneous generation of large volumes of gaseous hydrocarbons. The hydrocarbons and catalyst mixture flows out of the riser tube into a reactor vessel wherein the resultant gaseous hydrocarbons are taken off for distillation into various product fractions defined by boiling ranges. The spent catalyst is then separated in the reactor vessel and stripped of hydrocarbons by passing the catalyst through a stripper section which includes steam flowing up through the down-flowing catalyst usually for a period of 1 to 3 minutes. Catalyst is then returned to a regenerator where residual hydrocarbons, called "coke", on the spent catalyst are burned off by passing a stream of an oxygen-containing gas, such as air, or oxygen-enriched air, through the catalyst until substantially all the carbon is burned from the particles. The heat generated in this regeneration step is used as a heat source to heat the catalyst and thus provide elevated temperatures needed for reaction with the incoming hydrocarbon feed. Regenerated hot catalyst is then recycled to the riser cracking zone wherein the feed is cracked to form more gaseous products.
In recent years, the field of FCC has undergone significant development improvements due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalysts and particularly crystalline zeolite cracking catalyst, new areas of operating technology have been encountered requiring refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
Of particular interest has been the development of methods and systems for separating catalyst particles from a mixed phase containing catalyst particles and gaseous hydrocarbon products, particularly the separation of high activity crystalline zeolite cracking catalyst under more efficient separating conditions so as to reduce the overcracking of hydrocarbon conversion products and promote the recovery of desirable products of the hydrocarbon conversion operation. The separation of catalyst particles from a gaseous mixture is conventionally performed using a cyclone or a series of cyclones contained in the reactor vessel. However existing cyclonic equipment as conventionally used often permits an undesirable extended residence time of the product gases within a large reactor vessel. This extended residence time reduces the value of the product by non-selective thermal or uncontrolled catalytic cracking. Consequently, recent developments in this art are concerned with the rapid separation and recovery of entrained catalyst particles from the mixed phase.
Various processes and mechanical means have been employed heretofore to effect rapid separation of the catalyst from the hydrocarbons at the termination of the riser cracking zone in order to minimize contact time of the catalyst with cracked hydrocarbons.
The use of a simple riser cap over the riser reactor, i.e., in the form of a shroud, is very inefficient for the purpose of reducing product gas residence time. In such designs the reactor vessel is actually being used as an intermediate separator. A vented riser such as that disclosed in U.S. Pat. No. 4,701,307 is an attempt to improve on the limitations of the riser shroud.
U.S. Pat. No. 4,502,947 discloses a process in which a hydrocarbon gas and catalyst mixture passes directly from a riser reactor into a series of cyclone separators, which separate catalyst particles from the mixture and which adds stripping gas to the mixture as it passes from one cyclone separator to the next. Rapid separation is achieved by passing the mixture from the riser directly into a first cyclone through an enclosed conduit, thereby preventing the mixture of catalyst and gaseous products from filling the reactor vessel volume.
The problem of long vapor residence time in the reactor vessel which results in degradation of the product by thermal or uncontrolled catalytic cracking both of which are very non-selective, has been addressed by U.S. Pat. No. 4,502,947 by bypassing the dilute phase entirely. This patent discloses passing the mixture from the riser directly to a riser cyclone separator positioned within the reactor vessel. This can have a very beneficial effect on the yield. In particular, the amount of light gases (C.sub.2 and lighter) is greatly reduced. While the production of C.sub.3 and C.sub.4 gases also decreases, the gasoline production can rise by as much as 1.5% or more. Under these conditions the unit can be run at greater severity, i.e., higher temperature or more active catalysts to increase the production of C.sub.3 and C.sub.4. The net result is more total liquid of much higher octane and a more valuable product slate. The difficulty with the use of such a rough cut cyclone feeding directly from the riser is that the vertical height of the reactor internals give rise to a relatively short dipleg in the rough cut cyclone. The short dipleg can give rise to catalyst backup and eventual contamination of the fractionator, particularly when an existing reactor has been retrofit with a rough cut cyclone.
Consequently, it would be advantageous if a more compact separator could be configured within the reactor vessel to provide a longer dipleg preventing the danger of solids backup.
Other cyclonic separation methods of the prior art which address the same need for rapid separation of the catalyst phase from the hydrocarbon phase include U.S. Pat. Nos. 4,043,899; 3,661,799; 4,404,495; 4,591,427; 4,725,410; and 4,664,889; 4,295,961; 4,572,780; 4,588,558.
U.S. Pat. No. 4,070,159, provides a separation means whereby the bulk of catalyst solids is discharged directly into a settling chamber without passing through a cyclone separator. In this apparatus, the discharge end of the riser conversion zone is in open communication with the disengaging chamber such that the catalyst discharges from the riser in a vertical direction into the disengaging chamber which is otherwise essentially closed to the flow of gases. The cyclone separation system is in open communication with the riser conversion zone by means of a port located upstream from, but near, the discharge end of the riser conversion zone. U.S. Pat. No. 4,219,407, discloses the separation of a catalyst from the gas phase in a fashion which permits effective steam stripping of the catalyst. The suspension of catalyst and gaseous material is discharged from the riser outwardly through radially extending passageways, or arms, which terminate in a downward direction toward catalyst stripping zones. The arms are provided with curved inner surface and confining sidewalls to impart a centrifugally induced concentration of catalyst particles promoting a forced separation from the hydrocarbon vapors. U.S. Pat. No. 4,693,808 discloses an integral hydrocarbon conversion apparatus and process having a downflow hydrocarbon reactor, an upflow riser regenerator, and a horizontal cyclone separator. The horizontal cyclone separator can be equipped with a vortex stabilizer which acts to form a helical flow of vapors from one end of the cyclone separator to the hydrocarbon product outlet end. This vortex acts to separate entrained spent catalyst from the hydrocarbon product material. Other cyclonic means of vapor gas separation employing shrouds have been suggested, such as U.S. Pat. No. 4,591,427 which discloses a riser reactor feeding directly into a cyclone with a separate vapor recovery shroud through which the recovered vapors from the first cyclone are accessible to a cyclone separator. U.S. Pat. No. 4,404,095 minimizes the length of time the cracked vapors are exposed to the catalytic reaction product temperature by discharging the mixture outwardly through an opening in riser into a radial passageway discharging the catalyst particles downward and the separated gases into a cyclone separator.
The difficulty with all cyclonic separators fed directly from a riser, is that they have an inherently long vapor residence time in contact with the catalyst because they rely on helical vortex patterns to separate catalytic particles centrifugally. Indeed, cyclone separators have been sometimes used as gas-solids catalytic and noncatalytic reactors. It would be advantageous if initial separation could be effected by non-cyclonic means.
In connection with other hydrocarbon conversion processes relying upon the contacting of solids and hydro-carbonaceous vapors, inertial separators have been suggested, for example U.S. Pat. No. 4,318,800. This patent discloses a thermal regenerative cracking process. In this process hydrodesulfurized residual oil passes through a thermal cracking zone together with entrained inert hot solids functioning as a heat source and a dilute gas at a temperature between about 1300.degree. F. and 2500.degree. F., and a residence time between about 0.05 to 2 seconds to produce cracked product comprising ethylene and hydrogen.
While the latter patent is not concerned with catalytic processes, nor is it concerned particularly with fluidized bed catalytic cracking, it does disclose in FIG. 16 an interesting inertial separator for the separation of a particulate solid phase from a mixed gaseous-solid phase. The solids used in thermal regenerative cracking (TRC) processes are generally of different bulk density (heavier than FCC catalyst) and different average particle size (larger than FCC catalyst particles). For example, Example 1 of U.S. Pat. No. 4,318,800 discloses the TRC process separation of TRC silica alumina particles having an apparent bulk density of 70 lbs/cu ft. and an average particle size of 100 microns. FCC catalysts typically have an apparent bulk density of 40-50 lbs/cu ft. and a particle size of 50-80 microns diameter. FCC fines, which must also be separated, are even smaller particles. (Apparent bulk density is distinguished from compacted bulk density.) Where the TRC separator was tested with FCC catalyst, the TRC separator was not of FCC reactor scale nor operating conditions (col. 19). The disclosure of U.S. Pat. No. 4,318,800 is incorporated herein by reference.
U.S. Pat. Nos. 4,556,541 and 4,433,984 disclose that the same TRC inertial separator of substantially rectangular cross section provides a separation method and apparatus for essentially complete separation of TRC solids from the mixed phase stream. The prior art states that it is essential that the TRC separator flow path have a rectangular cross section in order to obtain good efficiency, i.e., U.S. Pat. No. 4,433,984 at column 3, line 60. The disclosures of U.S. Pat. Nos. 4,556,541 and 4,433,984 are incorporated herein by reference. Other art which has addressed the problem of TRC solid separation from gases within the context of thermal regenerative cracking include U.S. Pat. Nos. 4,338,187; 4,288,235; 4,348,364; 4,370,303; 4,061,562; 4,097,363; and 4,552,645. The TRC separator of U.S. Pat. No. 4,433,984 is said to be improved by the disclosed separator in U.S. Pat. No. 4,814,067, the improvement comprises a disengagement device modeled on the concepts of U.S. Pat. No. 4,288,235.
U.S. Pat. No. 4,640,201 describes a circulating fluidized bed furnace wherein a non-cyclonic particulate separator is integrally disposed in the gas flow path such that the momentum of the particulate solids and the centrifugal forces acting thereon prevent the solids from sharply turning and cause the solids to continue on their arcuate flow path towards the solids collection means.
U.S. Pat. No. 4,404,095 illustrates a conventional inertial separator. Essentially, the end of the riser makes a 90.degree. turn into a radial passageway and direct fluid communication with the inlet of a cyclone separator. In such a design however, there is low separation efficiency with resulting failure to achieve the desired objective.